First, make sure you have 3 stars. Then, enter the main hall and up the stairs ON THE RIGHT. Then enter the room. You'll see a giant blue painting ahead of you. Jump in.
Next, enter the water and swim forward till you see an empty, deep pool. Swim down and you should see a ship. You should also see an eel. Try luring it out of the hole its blocking. When it leaves, swim into the hole WITHOUT TOUCHING THE EEL.
When you enter the ship, you'll see chests. Open the chests in this order: Back, left, right, then front. In the chests are air bubbles you can inhale if your power is low. After following the pattern, the ship will drain. When its finished draining, climb the ledges to the top. You'll see a yellow ! box. Open the box to get the star.
I hope this helped!